vi PREFACE 
Now, after the lapse of seventeen years, it has been possible to state the 
biological argument more fully in the present volume, strengthened by many 
new facts. The First Part (pp. 1-254) deals with the general theory. The 
Second Part (pp. 255-657) is taken up with a detailed statement of the facts, 
together with comparison of the constituents of the several phyla zzder se. 
The Third Part (pp. 658-717) is devoted to general comparisons and con- 
clusions. The attempt has been made to work in the results of Palaeonto- 
logical research with those of the comparative analysis of living forms. The 
enquiry has related to all the characters, both vegetative and propagative, of 
the sporophyte generation: these include the external form, the embryogeny, 
and anatomical features, and especially the structure and development of 
the Spore-producing members, while the characters of the gametophyte 
have also been taken into account. It is found that the conclusions 
arrived at are supported by general convergence of the lines of evidence 
derived from all of these sources. 
The method adopted in the preparation of this work has been to 
examine not only the mature structure, but also the development of the 
organisms, and of their several parts. While fully utilising the results of 
Palaeontological and anatomical study, considerable weight has throughout 
been given to the facts of the individual development: sometimes the latter 
appear to oppose the former. It is not held that the ontogenetic history 
will always serve as an infallible guide, and opportunity has been taken 
to point out that conclusions based upon it are liable to be overruled by 
the results of wide comparison (pp. 159, 636, and 660, footnote). But it 
is felt that in much of the recent work on Pteridophytes, and especially 
where fossil comparisons come_in, the arguments from individual develop- 
ment have been accorded less than their due share of attention. 
I have made no attempt to give comprehensive or complete biblio- 
graphical references: from Campbell’s Aosses and Ferns and from other 
sources such references can readily be obtained. But wherever a quotation 
is made, or where a substantial body of information derived from another 
author has been embodied in the text, the reference is fully given. While 
thus acknowledging my indebtedness to those whose work is published, 
I desire also to record the continuous personal help so willingly given by 
three friends and colleagues, who have all allowed me «the use of unpub- 
lished drawings and facts. Mr. Kidston’s peculiarly exact knowledge has 
greatly strengthened and amplified the Palaeontological statements, while 
Dr. Lang and Mr. Gwynne-Vaughan have given me throughout the 
assistance of friendly criticism, and the support of their special knowledge 
of certain branches of the matter in hand. 
In conclusion, I am well aware that the chief question dealt with 
